Volume 56, Issue 4, July 2012
Index of content:
56(2012); http://dx.doi.org/10.1122/1.4708515View Description Hide Description
In geometries capable of producing uniform shear stress across the gap such as cone–plate, it is a challenge to study the nonlinear rheology of well-entangled polymers without wall slip and edge fracture. The task to obtain meaningful steady-state information is particularly daunting. Significant insights are required in the design of experiments and choice of reliable apparatuses that could fulfill the objective to determine the constitutive behavior. In a recent published article, Hu J. Rheol. 54, 1307–1323 (2010) reported particle-imaging velocimetric measurements of two highly entangled polybutadiene solutions with a cocylinder geometry. One of the solutions was prepared and provided to Hu by Ravindranath from the University of Akron. Rheological and particle-tracking velocimetric studies have been carried out on the same sample in our laboratory. In this letter, we will compare Hu’s results with our own measurements, clarify a few misrepresentations, and discuss several important issues concerning shear banding in entangled polymers.
56(2012); http://dx.doi.org/10.1122/1.4708340View Description Hide Description
The rheology of slurries prepared from artificial mixtures of bitumen with fine quartz of varying bitumen content, as well as slurries of actual oil sand ores, was investigated under different physicochemical conditions (pH and temperature). All rheological measurements were conducted using a Haake Rotovisco VT550 rotational viscometer, connected to a fixture specifically designed to measure the rheological properties of settling suspensions. It was found that when the bitumen content in slurries prepared from quartz-bitumen mixtures was low (0.7 vol. %), the pH of the slurries was the most important parameter that controlled the rheology of these slurries. At higher bitumen contents (8 vol. %), it was a combination of high temperature and high pH that gave the lowest apparent viscosities and yield stresses of the model mixtures. These model observations correlated well with the rheology data obtained for slurries of actual oil sand ores. The overall results were discussed in terms of aggregation-dispersion phenomena between bitumen and mineral particles.
56(2012); http://dx.doi.org/10.1122/1.4708594View Description Hide Description
Recent atomistic simulations of polyethylene melt afford a close look at the entanglement dynamics of a real polymer. We analyze these results in a new way, by defining the tube primitive path as the mean path of consecutive molecular dynamics trajectories. The result suggests that tube is semiflexible, the persistence length being about half the entanglement length. The time dependent tangent–tangent correlation function of tube primitive path is then used to test the standard molecular model of tube dynamics. It is found that the effect of the semiflexibility is important in the mildly entangled system we studied and that incorporating its effect into the standard tube model improves the theory drastically.
Time evolution of the structure of organoclay/polypropylene nanocomposites and application of the time-temperature superposition principle56(2012); http://dx.doi.org/10.1122/1.4708602View Description Hide Description
We investigated the rheological properties of nanocomposites composed of polypropylene, organoclay, and maleic anhydride grafted polypropylene in small amplitude oscillatory shear. Samples were prepared in two steps: a masterbatch was first obtained by melt extrusion and then diluted into polypropylene using an internal mixer. Three formulations were investigated. The measurement of the storage and loss moduli evolution with time showed that these materials were not stable: the nanostructure obtained after steady shear continuously changed with time, due to the disorientation of the clay platelets and the build-up of a 3D network. The kinetics of the structure build-up (followed via the melt yield stress) showed a two-step process. This feature was found to be valid whatever the nanocomposite formulation. Such evolution of the structure is generally assumed to violate the time–temperature superposition principle. We demonstrate in this paper that the time–temperature equivalence always exists if the same nanostructure is probed. This was achieved by using different annealing times for different temperatures or annealing the samples at the highest temperature before measuring at lower values. The time–temperature equivalence evidences that the temperature does not induce any chemical change within the material, whose properties remain governed by the same physical phenomena.
Viscoelastic properties of solutions of polystyrene melts and carbon dioxide: Analysis of a transient shear rheology approach56(2012); http://dx.doi.org/10.1122/1.4708601View Description Hide Description
In this work, the viscoelastic, thermal, and diffusion properties of solutions of polystyrene melts and carbon dioxide (CO2) were analyzed using plate–plate rheometry in the transient mode. The objective of this study was to evaluate a transient shear rheology approach for high viscositypolymer melts, to verify superposition principles for polystyrene/CO2 solutions, and to measure the glass transition temperature as a function of pressure and CO2 concentration. Two different procedures of saturating polystyrene with carbon dioxide were applied, i.e., loading with the blowing agent below the glass transition temperature of polystyrene and at the measurementtemperature. Stress-growth experiments in shear were performed in order to measure the transient viscosity of polystyrene/CO2 solutions in the linear regime. A shift of the transient viscosity data to a mastercurve was applied in order to determine the shift factor of the viscosity and the average relaxation time. Our data indicate that the steady-state viscosity and the average relaxation time are proportional to the temperature–pressure–concentration shift factor within experimental scatter, and consequently a time–temperature–pressure–concentration superposition principle holds for polystyrene/CO2 solutions. Whereas the viscosity of polystyrene strongly depends on the applied temperature and pressure, the elastic equilibrium compliance depicts only a weak dependence on temperature and pressure.
56(2012); http://dx.doi.org/10.1122/1.4710528View Description Hide Description
It has recently been shown that the dynamics of complex interfaces can be treated by hyperelastic finite deformation, and the Cauchy stress tensor can be written as a tensor derivative of the interfacial energy. In the present study, the analytical nature of this tensor derivative was further explored, resulting in a more useful constitutive relation between the stress tensor and the Finger strain tensor. To accommodate the interfacial relaxation effects, an energy balance principle was adopted in developing a nonlinear relaxation model which was then used in a visco-hyperelastic formulation for stress determination. Case studies on this visco-hyperelastic model were performed, and validations against known results were attempted. While at small deformation rates, the new model agrees with the linear viscoelastic calculations, as well as Doi and Ohta’s scaling predictions, high nonlinearity was observed at large deformation rates.
Microrheology, microstructure, and aging of physically cross-linked poly(vinyl alcohol)/poly(ethylene glycol) blends56(2012); http://dx.doi.org/10.1122/1.4708603View Description Hide Description
Physically cross-linked blends of concentrated poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) undergo microphase separation and gel as they age. We investigate the microrheology and microstructure of these materials by using particle tracking and dynamic light scattering to measure the thermal motion of small polystyrene spheres suspended in the blends. Dynamic light scattering probes the ensemble-averaged motion of all the probe particles over a wide range of time scales, while video-based particle tracking follows the motion of many individual tracer particles. Dynamic light scattering shows that the tracer particles move diffusively at short and long time scales, while their motion is restricted by materialelasticity at intermediate time scales. The particle tracking experiments show a wide distribution of mean square displacements at a given lag time, indicating spatial heterogeneity on the micron length scale. We separate the particles into three populations according to their behavior. We extract information about the local microrheological environments probed by each population and study their dependence on PEG concentration and aging time. We find that addition of PEG to the PVA solutions influences the microrheological environment significantly, and that phase separation occurs prior to gelation as the blends age. The gelation time determined from the microrheological measurements is later than the bulk gel time, indicating that the different phase-separated regions age differently. Our results are consistent with a model in which the PVA/PEG blends consist of PVA-poor pores within a continuous PVA-rich domain.
56(2012); http://dx.doi.org/10.1122/1.4707948View Description Hide Description
We develop a full-chain tube-based constitutive model [along the lines of Graham et al. J. Rheol. 47, 1171 (2003)] for the nonlinear rheology of bidisperse blends of long and short linear polymers. For a test chain in the blend, we use the physical picture of a fat tube, representing long-lived entanglements with long chains, and a thin tube, representing entanglements with all chains. The model includes the processes of reptation, contour length fluctuation (CLF), constraint release, and stretch relaxation. In the linear rheology regime, we identify a new relaxation process: CLF along the fat tube contour, achieved via a combination of chain motion along the thin tube, and local constraint release of the thin tube as it explores the width of the fat tube. This process is sufficiently fast to relax a significant portion of the long chains before reptation. It provides an explanation of the decrease in terminal time of long chains upon dilution with short chains in a framework where motion along the thin tube is the dominant reptation mechanism. Once the linear rheology is matched, nonlinear rheology is predicted with no further adjustments to the model. The model compares well against several experimental datasets on bidisperse blends. In particular, it predicts the onset rate of extension hardening, which is often significantly below the inverse Rouse time of the long chains.
56(2012); http://dx.doi.org/10.1122/1.4709423View Description Hide Description
Many densely packed suspensions and colloids exhibit a behavior known as Discontinuous Shear Thickening in which the shear stress jumps dramatically and reversibly as the shear rate is increased. We performed rheometry and video microscopy measurements on a variety of suspensions to determine the mechanism for this behavior. We distinguish Discontinuous Shear Thickening from inertial effects by showing that the latter are characterized by a Reynolds number but are only found for lower packing fractions and higher shear rates than the former. Shear profiles and normal stress measurements indicate that, in the shear thickening regime, stresses are transmitted through frictional rather than viscous interactions. We come to the surprising conclusion that for concentrated suspensions such as cornstarch in water which exhibit the phenomenon of Discontinuous Shear Thickening, the local constitutive relation between stress and shear rate is not necessarily shear thickening. If the suspended particles are heavy enough to settle, we find the onset stress of shear thickening corresponds to a hydrostatic pressure from the weight of the particle packing where neighboring particles begin to shear relative to each other. Above , dilation is seen to cause particles to penetrate the liquid–air interface of the sheared sample. The upper stress boundary of the shear thickening regime is shown to roughly match the ratio of surface tension divided by a radius of curvature on the order of the particle size. These results suggest a new model in which the increased dissipation in the shear thickening regime comes from frictional stresses that emerge as dilation is frustrated by a confining stress from surface tension at the liquid–air interface. We generalize this shear thickening mechanism to other sources of a confining stress by showing that, when instead the suspensions are confined by solid walls and have no liquid–air interface, is set by the stiffness of the most compliant boundary which frustrates dilation. All of this rheology can be described by a nonlocal constitutive relation in which the local relation between stress and shear rate is shear thinning, but where the stress increase comes from a normal stress term which depends on the global dilation.
56(2012); http://dx.doi.org/10.1122/1.4717494View Description Hide Description
This paper is concerned with the rheology of algae suspensions relevant to algae biofuel processing for a range of concentrations up to 15 vol. % using mostly a piezoaxial vibrator (PAV) rheometer as a method of measuring rheological properties. Linear viscoelastic (LVE) measurements of a Scenedesmus obliquus [culture collection of algae and protozoa (CCAP) 276/7] living algae strain were obtained and a curve for complex viscosity (η*) as a function of concentration/volume fraction derived. The PAV complex viscosity data increased exponentially with cell concentration and elasticity (G′) developed in a similar way with increasing concentration. The results indicated the presence of interaction between algae cells at all measured concentrations. For concentrations above ∼5 vol. %, steady shear data obtained using a Couette geometry showed non-Newtonian “shear-thinning” behavior and at higher concentrations there was a divergence from the Cox–Merz rule. A difference in the LVE rheological measurements was found for cells that were either alive or dead indicating that cell motility and significant interparticle contact and interactions influenced levels of viscoelasticity. The results are of potential scientific relevance and also useful in relation to the design of algae bioprocessing for the production of biofuels.
56(2012); http://dx.doi.org/10.1122/1.4709431View Description Hide Description
Rheology and viscosity data of liquid metals in high temperatures are important in metallurgical manufacturing processes. In this paper, a simple and low cost viscometer which can be used to measureviscosity of liquid metals and semisolid slurries in different shear rates is introduced. In this apparatus, the principles of the falling ball viscometer are used while accumulated with a counterbalance mass for decreasing ball speed and controlling shear rates. The accuracy of the instrument is demonstrated by using liquids with known rheology. The obtained data were comparable with the available results in the literature.
Using startup of steady shear flow in a sliding plate rheometer to determine material parameters for the purpose of predicting long fiber orientation56(2012); http://dx.doi.org/10.1122/1.4717496View Description Hide Description
The properties of long glass fiber reinforced parts, such as those manufactured by means of injection molding and compression molding, are highly dependent on the fiber orientation generated during processing. A sliding plate rheometer was used to understand the transient stress and orientation development of concentrated long glass fibers during the startup of steady shear flow. An orientation model and stress tensor combination, based on semiflexible fibers, was assessed in its ability to predict fiber orientation when using model parameters obtained from the fits of the stress responses. Specifically, samples of different initial fiber orientations was subjected to the startup of steady shear flow, and an orientation model based on bead and rod theory was coupled with a derived stress tensor that accounts for the semiflexibility of the fibers to obtain the corresponding model parameters. The results showed the semiflexible orientation model and stress tensor combination, overall, provided improved rheological results as compared to the Folgar–Tucker model when coupled with the stress tensor of Lipscomb et al. [J. Non-Newtonian Fluid Mech. 26, 297–325 (1988)]. Furthermore, it was found that both stress tensors required empirical modification to accurately fit the measured data. Finally, orientation models provided encouraging results when predicting the transient fiber orientation for all initial fiber orientations explored.